1. Field of the Invention
This invention relates to a process for applying a film to a relief substrate, and, more particularly to a process for applying a photoresist film to a preimaged patterned relief substrate by flooding the substrate to cover the relief prior to application of the photoresist film.
2. Description of the Prior Art
The application of dry film resist materials to substrates, such as printed circuit boards, is known. Typical substrates, e.g., copper clad epoxy or phenolic boards, metal clad ceramic substrates, etc., are substantially flat and smooth prior to processing except for microscopic surface irregularities or scratches resulting from machining or manufacture. Typically, dry film resists are applied to the aforementioned substrates by lamination on a roll-type laminator at elevated temperature to effect heat and pressure bonding and to achieve intimate contact on a microscopic scale between the photoresist and the microscopically irregular substrate surface. The bond obtained is satisfactory and practical for these applications and, in fact, is standard in the printed circuit industry.
U.S. Pat. No. 3,629,036 discloses an alternate method of applying photoresists to substrates which has some advantages over hot laminations. In the described process, a very thin coating of liquid adhering agent 10 to 20 millionths of an inch (0.00025-0.00051 mm) thick, which preferably is a solvent for the resist and contains a small amount of dissolved resist, e.g., 0.5 to 5.0% by weight, is applied to the substantially flat substrate surface, and the dry film resist is then applied whereby the resist surface becomes softened and adhered to the substrate. This method eliminates the use of heat which may damage the dry film resist structure or its support film for certain selected compositions or film structures. The need for substantial pressure during lamination is also reduced. Dissolved resist is stated to be present to provide a fill for the microscopic surface irregularities and to eliminate minute voids of trapped air pockets which may degrade the bonding and permit seepage of etchant. While the described process may be advantageous for those photoresists where heat and pressure bonding does not provide enough plasticity and flow to achieve intimate contact of the relatively thick resists, e.g., 0.0005 to 0.005 inches (0.013 to 0.13 mm) and more over the minute irregularities (resist thickness to irregularity height ratio is 50:1 or greater), most commercial photoresist films and heated roll laminators achieve adequate bonding so that hot roll lamination is the preferred standard practice for applying dry film photoresists.
Often it is desired to laminate dry film resists to substrates that contain relief images on their surfaces as opposed to the substantially flat substrates discussed above. These substrates are of the same or similar materials as described above but contain on their surface patterned relief images in the form of completed or partially completed circuits or circuit elements or other functional or decorative relief. For example, completed printed circuits contain metallic relief image heights of 0.001 to 0.005 inches (0.025 to 0.13 mm) or greater for macroelectronic uses and 0.000045 to 0.0001 inch (0.001 to 0.0025 mm) for microelectronic uses. Often photoresist films of thickness comparable to the relief height or less (resist thickness to relief height ratio of 2:1, 1:1 or less) are laminated to these relief surfaces. In such instances, lamination with standard hot roll laminators often does not allow the resist to conform perfectly around the upraised relief. Instead the resists tend to "tent" over the valleys between the upraised relief trapping large air pockets which subsequently interfere with proper resist exposure and development. The resist tenting over the open spaces is fragile, unsupported resist which is easily broken or damaged, and, more importantly, permits processing solutions, such as etchants or developers, to penetrate far under the resist in large amounts to act on circuit elements that should be protected by the resist from the action of these solutions.
Several methods have been devised to eliminate or reduce this problem by permitting resist to conform more completely to the relief images during lamination. One method, taught in U.S. Pat. No. 3,984,244, achieves improved conformation and substantial elimination of air entrapment by placing grooves in the photoresist just prior to lamination under heat and pressure. The grooves permit air to escape along them thereby substantially eliminating air entrapment. Another method utilizes vacuum lamination in a chamber purged of air to low pressure, followed by heat and pressure lamination, thus greatly reducing trapped air. These two methods are most useful where the resist is thicker than the height of relief.
It has now been found that by the process of the invention, as described below, excellent photoresist conformation to raised relief images ranging from 0.00004 inch (0.001 mm) up to 0.005 inch (0.13 mm) or greater in height is achieved using photoresist films with thicknesses comparable to or less than that of the relief height. The process of invention also prevents the entrapment of air under the laminated resist thereby eliminating the penetration of processing solutions under the exposed and developed images.